A second man has been arrested in connection with the Andrew Mitchell "plebgate" affair, police have said.

The 23-year-old, who is not a police officer or a member of police staff, was arrested on Wednesday night on suspicion of intentionally encouraging or assisting the commission of an indictable offence on or around December 14, Scotland Yard said.

He was questioned at a London police station and released on bail to return in January, a spokesman said.

In a statement, Scotland Yard said: "Officers investigating the circumstances surrounding a police officer's claim to have witnessed an incident in Downing Street in September 2012 have arrested a man on suspicion of intentionally encouraging or assisting the commission of an indictable offence on or around December 14 2012."

On Wednesday, police widened the investigation into the affair amid signs of strain between senior Conservatives and the police over the treatment of the former chief whip.

Some 30 officers are working on the inquiry - which is being supervised by the Independent Police Complaints Commission (IPCC) - amid claims that officers conspired to fabricate evidence against Mr Mitchell that ultimately led to his resignation in late October.

Prime Minister David Cameron has expressed concern that an officer tried to "blacken the name" of Mr Mitchell amid mounting questions over the initial account of his row with Downing Street police who refused to let him ride his bicycle through the main gates of Downing Street.

The officer is said to have written an email to his local MP, Conservative John Randall, posing as a member of the public and accusing Mr Mitchell of calling police "plebs". The account contained in the email, written the day before the Sun first broke news of the row on September 21, was very similar to that in the police log, which was later leaked to the Daily Telegraph.

Scotland Yard has said it was conducting an investigation which could look at the possibility of a conspiracy. In a statement, it said: "The allegation that a serving police officer fabricated evidence is extremely serious. It goes to the very heart of the public's trust in the police service."

An officer was arrested on Saturday on suspicion of misconduct in public office.